stages: add kernel-cmdline.bls-append stage

This adds a stage to be able to add kernel arguments on a system by
appending to the BLS [1] config directly either in the tree or in
a mount. This is useful on say systems that don't use `grubby` and
thus can't use the org.osbuild.kernel-cmdline stage.

[1] https://freedesktop.org/wiki/Specifications/BootLoaderSpec/

stages/org.osbuild.kernel-cmdline.bls-append

@@ -0,0 +1,86 @@
+#!/usr/bin/python3
+"""
+Add kernel command line parameters to a BLS [1] config either in
+the tree or in a mount.
+
+[1] https://freedesktop.org/wiki/Specifications/BootLoaderSpec/
+"""
+
+
+import glob
+import sys
+from urllib.parse import urlparse
+
+import osbuild.api
+
+SCHEMA_2 = r"""
+"options": {
+  "additionalProperties": false,
+  "required": ["kernel_opts"],
+  "properties": {
+    "kernel_opts": {
+      "description": "Additional kernel command line options",
+      "type": "array",
+      "items": {
+        "description": "A single kernel command line option",
+        "type": "string"
+      }
+    },
+    "bootpath": {
+      "type": "string",
+      "description": "The mounted location of the boot filesystem tree where the BLS entries will be under ./loader/entries/*.conf",
+      "pattern": "^(mount|tree):\/\/\/",
+      "examples": ["tree:///boot", "mount:///", "mount:///boot"],
+      "default": "tree:///boot"
+    }
+  }
+},
+"devices": {
+  "type": "object",
+  "additionalProperties": true
+},
+"mounts": {
+  "type": "array"
+}
+"""
+
+
+def main(paths, tree, options):
+    kopts = options.get("kernel_opts", [])
+    bootpath = options.get("bootpath", "tree:///boot")
+
+    url = urlparse(bootpath)
+    scheme = url.scheme
+    if scheme == "tree":
+        root = tree
+    elif scheme == "mount":
+        root = paths["mounts"]
+    else:
+        raise ValueError(f"Unsupported scheme '{scheme}'")
+
+    assert url.path.startswith("/")
+    bootroot = root + url.path
+
+    # There is unlikely to be more than one bls config, but just
+    # in case we'll iterate over them.
+    entries = []
+    for entry in glob.glob(f"{bootroot}/loader/entries/*.conf"):
+        entries.append(entry)
+        # Read in the file and then append to the options line.
+        with open(entry, encoding="utf8") as f:
+            lines = f.read().splitlines()
+        with open(entry, "w", encoding="utf8") as f:
+            for line in lines:
+                if line.startswith('options '):
+                    f.write(f"{line} {' '.join(kopts)}\n")
+                else:
+                    f.write(f"{line}\n")
+    assert len(entries) != 0
+    print(f"Added {','.join(kopts)} to: {','.join(entries)}")
+    return 0
+
+
+if __name__ == '__main__':
+    args = osbuild.api.arguments()
+    r = main(args["paths"], args["tree"], args["options"])
+    sys.exit(r)